1. Field of the Invention
The present invention relates to digital video streaming, and more particularly to a method and system for video stream encryption.
2. Description of the Related Art
Digital video stream encryption systems have been used for access authentication. FIG. 1 is a diagram of a conventional digital video stream encryption system. An encryption application 11 receives video data video 1 to encrypt and outputs an encrypted video stream video 2 to a corresponding decryption application 12 via various media, such as microwave, internet, or cable. The decryption application 12 is responsible for decrypting the encrypted video stream video 2 to restore the original video stream video 1.
A digital video stream can be seen as a series of static frames, requiring considerable storage capacity and transmission bandwidth. A 90-min full color video stream, for example, having 640×480 pixels/frame and 15 frames/second, requires bandwidth of 640×480 (pixels/frame)×3(bytes/pixel)×15(frames/sec)=13.18(MB/sec) and file size of 13.18(MB/sec)×90×60=69.50(GB). Such a sizeable digital video stream is difficult to store and transmit in real time, thus, many compression techniques have been introduced.
MPEG standards ensure video encoding systems create standardized files that can be opened and played on any system with a standards-compliant decoder. Digital video contains spatial and temporal redundancies, which may be compressed without significant sacrifice. MPEG coding is a generic standard, intended to be independent of a specific application, involving compression based on statistical redundancies in temporal and spatial directions. Spatial redundancy is based on the similarity in color values shared by adjacent pixels. MPEG employs intra-frame spatial compression on redundant color values using DCT (Discrete Cosine Transform) and quantization. Temporal redundancy refers to identical temporal motion between video frames, providing smooth, realistic motion in video. MPEG relies on prediction, more precisely, motion-compensated prediction, for temporal compression between frames. MPEG utilizes, to create temporal compression, I-Frames, B-frames and P-frames. An I-frame is an intra-coded frame, a single image heading a sequence, with no reference to previous or subsequent frames. MPEG-1 compresses only within the frame with no reference to previous or subsequent frames. P-frames are forward-predicted frames, encoded with reference to a previous I- or P-frame, with pointers to information in a previous frame. B-frames are encoded with reference to a previous reference frame, a subsequent reference frame, or both. Motion vectors employed may be forward, backward, or both.
MPEG achieves compression by quantizing the coefficients produced by applying a DCT to 8×8 blocks of pixels in an image and through motion compensation. Quantization is basically division of the DCT coefficient by a quantization scale related to quality level, with higher indices for greater compression but lower quality, and lower indices for the reverse.
In the past, conventional encryption techniques have normally encrypted entire compressed video stream, as have conventional decryption techniques. Several inherent limitations exist in this process. First, encrypted video stream is unreadable without corresponding decryption, such that preview is unavailable. In addition, much time is spent encrypting and decrypting the entire video stream.
In view of the limitations described, a need exists for a system and method of video stream encryption to provide both low quality digital video for preview and high quality encrypted stream for subsequent decryption, with reduced time spent encrypting and decrypting.